The present invention relates generally to semiconductor wafer inspection, and more specifically to electron spectroscopic imaging techniques.
Semiconductor fabrication technology produces devices that have circuit structures with extremely small critical dimensions. Semiconductor wafer inspection systems require a high level of resolution in order to determine the integrity of these extremely small structures. One common inspection technique involves the use of an electron beam, for example, within a scanning electron microscope. However, the capabilities of these techniques are reaching the limit of their usefulness when considering the continually shrinking size of integrated circuit critical dimensions. As a result, the resolution of these techniques may not be high enough to provide useful inspection data.
Enhancement of spatial resolution , as well as useful physical or chemical information, can be achieved through electron energy discrimination, i.e. electron spectroscopic imaging (L. Reimer xe2x80x9cImage Formation in Low-Voltage Scanning Electron Microscopyxe2x80x9d, Ed. SPIE Optical Engineering Press, 1993, p. 2). In fact, electrons with different energy are representative of different interaction volumes within the sample. By selecting the proper energy range, it is then possible to minimize the beam-sample interaction volume, thus improving the spatial resolution of an electron microscope.
Generally, an inspecting electron beam that is directed at a spot on a semiconductor wafer causes electrons to scatter from the spot. FIG. 1 illustrates a generic electron energy spectrum 100 for electrons that are caused to scatter from a semiconductor wafer due to an electron beam. As it is commonly known, both secondary electrons 102 and back-scattered electrons 104 can be useful for specific inspection techniques. Energy level 106 represents the maximum energy level detected by the inspection machine, which is equivalent to the energy level of the electron beam.
Various spectroscopy techniques are known for selecting a certain region in the energy spectrum during inspection processes. One known technique utilizes a dispersive element, for example, a magnetic field. A dispersive element separates an electron beam into a spectrum of its various component energy levels. This is analogous to a prism that separates white light into a spectrum of primary colors. Detectors can then be used to detect the electron intensity levels at desired ranges of energy. Unfortunately, these techniques are generally difficult and expensive to built.
In view of the foregoing, a spectroscopic technique that is easy to implement, cost effective, and that provides a high spectral resolution would be desirable.
The present invention pertains to a simple and cost effective technique for obtaining spectral information in an electron microscope. Some applications of this technique are high-resolution imaging and elemental mapping. The technique involves a high pass energy filter that is alternately set, or multiplexed, at two energy levels. For an inspected area on a wafer, the detected intensity level at the higher energy setting is subtracted from the intensity level at the lower energy setting. The resulting intensity level (the differential value) corresponds to the detected electron energy within the energy range of the first and second filter settings. Accordingly, images of the wafer for specific energy ranges may then be obtained
One aspect of the present invention pertains to a high spatial resolution method for inspecting a specimen by detecting electrons that scatter from the specimen. This method includes scanning and directing an electron beam to irradiate a spot on the specimen, the electron beam causing the electrons to scatter from the irradiated spot on the specimen. A high pass filter is then set at a first voltage level. Then the scattered electrons are detected with the high pass filter to measure a first electron intensity level. Then the high pass filter is set at a second voltage level. Then the scattered electrons are detected with the high pass filter to measure a second electron intensity level. A differential electron intensity level, which is the difference between the first electron intensity level and the second electron intensity level is then determined. The differential electron intensity level is the electron intensity level in an energy window between the first and second voltage level. By using this technique, the spatial resolution of the electron microscope can be enhanced by appropriately selecting the energy window of the detected electrons.
An alternative application of the method is used to identify an interface between two different materials on the specimen. Yet another alternative application of the method is used to perform spectroscopy on the specimen.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.